Not a fair fight since One Punch Man can beat anyone with one punch. It doesn't matter if it is everyone from the MCU. It will just take Saitama longer to beat them all. He is not meant to be taken seriously so his powers are exaggerated.

while i understand that one punch man is more satirical so his powers are exaggerated, The mcu's more powerful characters, Like galactus and the living tribunal would easily be able to kill One Punch man. And if you include the One above all then it is nowhere near a fair fight